Framework for thinking about tachycardia
Supraventricular tachycardia. AV non-nodal reentrant tachycardia. SV tachycardia with aberrancy such as a bundle branch block. Bundle who? Super what?
Tachycardia is guilty of committing the sin of medicalese. An overuse of lengthy latin and greek words which make the topic appear unattainable. I certainly felt this way until I came across references that view tachycardias through a system. Although there are a few of these resources that are great (i.e. Rob Theriault’s CDI) my personal favourite is Ed Wallit’s of PodMedics.
The most common tachycardia (any rhythm at, or over, 100 bpm) is a sinus one. Sinus tachycardia can arise secondary to a few conditions including a normal physiological response to stress. The above chart is from PodMedic’s 2009 podcast on tachycardia and if you have not registered (free) with them I encourage you to do so. The chart represents a decision tree one can move through when analyzing a tachycardic EKG.
The first step is to look at the QRS complexes and discern whether they are narrow (less than 0.12 s) or broad (at or longer than 0.12, or three small squares). Some may think the term narrow-complex refers to a pathological state, this is not the case. An electrical wavefront can only move through the heart faster than 0.12 s if the normal conduction pathways are intact and functioning. If the QRS complexes are narrow then the origin of the rhythm, and potentially the problem, is upstairs in the atria – or supraventricular.
If the QRS complexes are narrow the next question is whether the rhythm is regular. An irregularly irregular (that never gets easier to pronounce) rhythm is a hallmark indicator of atrial fibrillation. As Rob Theriault points out, rhythm irregularity is a stronger indicator of AF than a chaotic baseline as the latter could be artifact. If the rhythm is regular you have three likely candidates: sinus tach, AVNRT and AVRT keeping in mind that rhythms over 160 bpm are rarely sinus. AVNRT and AVRT are two rhythms guiltiest of medicalese. Have a look at this post for a quick and simple overview.
If the QRS complexes are wide (or broad, PodMedics is British) then we are having problem. With one exception broad QRS tachycardias can be ventricular tachycardia, ventricular fibrillation or torsades des pointes. Ventricular tachycardia is a very fast rhythm that originates in the ventricles instead of the SA node. The patient is at risk because the heart may not have sufficient fill time for effective systemic circulation and also because Vtach can very quickly degenerate into ventricular fibrillation. Vfib is a useless quivering of the heart. Electrical activity is completely unorganized and the heart is not pumping blood into the body. Death can occur within minutes. Torsades des pointes is french for twisting of the points. Torsades is similar to Vtach except the morphology (what the waveforms look like) changes. It is also treated differently, usually with IV magnesium in a stable Px.
There is a caveat (there is always a caveat) to the above. Not every wide complex tachycardia is of ventricular origin, sometimes it can still be the annoying neighbours upstairs. If a patient also had a bundle branch block, which would slow down the normal conduction pathway, an SVT could present with widened QRS complexes. This is termed SVT with aberrancy (fancy word for different).